The following publications are representative of the most relevant prior art known to the Applicants at the time of filing of the application.
______________________________________ UNITED STATES PATENTS ______________________________________ 1,338,598 April 27, 1920 C. W. Thomas 1,918,312 July 18, 1933 C. E. Wooddell 4,314,827 February 9, 1982 M. A. Leitheiser et al. 4,543,107 September 24, 1985 C. V. Rue 4,623,364 November 18, 1986 T. E. Cottringer et al. 4,741,743 May 3, 1988 K. S. Narayanan 4,744,802 May 17, 1988 M. G. Schwabel ______________________________________
The most significant development in the abrasive industry in recent years is a new type of non-fused or sintered abrasive with properties different from those of other abrasives. The unique properties of this new abrasive are primarily the result of the microstructure of the abrasive which in turn is a result of the processing techniques used to manufacture the material. One abrasive of this type is disclosed in U.S. Pat. No. 4,623,364. The product is, basically, a sintered aluminous abrasive which is highly dense and has a hardness of at least 18 GPa made up of a plurality of microcrystals of alpha alumina which are generally equiaxed and have a crystal size of no greater than 0.4 microns. This ultra finely crystalline alumina is prepared by forming an aqueous sol from water, finely pulverized, i.e. microcrystalline hydrated alumina, and a mineral acid; the sol may also contain varying amounts of zirconia or spinel forming magnesia. To the sol is added an effective amount of submicron alpha alumina particles which will function as seeds or a nucleating agent when the sol is fired at elevated temperature. The sol is cast into sheets or extruded, dried, and granulated. The green granules are then fired at about 1400.degree. C.
Another sintered aluminous abrasive is that taught by U.S. Pat. No. 4,314,827, the major difference being this method does not include the addition to the sol of sub micron alpha alumina seed material. Here too, however, the composition may include other materials such as zirconia, hafnia, or mixtures of the two, or a spinel formed from alumina and an oxide of cobalt, nickel, zinc, or magnesium. Abrasive grain made in this manner contains alpha alumina in the form of cells or sunburst shaped alpha alumina crystals having a diameter of 5-15 microns, is somewhat lower in density than the preceding abrasive, and has a density of only about 15 GPa.
U.S. Pat. No. 4,744,802 also describes a seeded sol gel sintered aluminous abrasive which is seeded by alpha ferric oxide or alpha alumina particles. The product is made by preparing a sol of alpha alumina monohydrated particles, gelling the sol, drying the gel to form a solid, and sintering the calcined gel.
There are, of course, other sintered abrasives that have been in commerce for years, such as abrasives based on sintered bauxite and sintered alumina-zirconia.
While sintered aluminous abrasives have properties that should make them outstanding abrasives, they do not live up to expectations in two significant areas. One area is in dry grinding with wheels wherein the abrasive is bonded with the more commonly used vitrified bonds, i.e. those that are fired and matured at temperatures of about 220.degree. C. or above. As stated in U.S. Pat. No. 4,543,107, attempts to use sintered aluminous abrasive bonded with such vitrified bonds in dry grinding were not completely successful. This is completely contrary to what happens with abrasive products bonded with so-called resinoid or organic polymer bonds; these bonds mature at temperatures in the range of 160.degree. C. to 225.degree. C. The same is true when the sintered aluminous abrasives are used in coated abrasive products. Organic bonded grinding wheels are exemplified in U.S. Pat. No. 4,741,743. A seeded sol gel type abrasive as described in U.S. Pat. No. 4,623,364 is bonded with a phenol-formaldehyde type bond, in combination with a co-fused alumina-zirconia abrasive. The unique properties of the seeded sol gel sintered aluminous abrasive in combination with the cofused alumina-zirconia produce a synergistic effect and result in cut-off wheels with grinding qualities or ratios significantly superior to wheels containing the seeded sol gel sintered aluminous abrasive alone or the cofused alumina-zirconia alone. Under two sets of grinding conditions, the wheels containing sintered aluminous abrasive alone were superior to wheels containing the heretofore superior cofused alumina-zirconia abrasive; in one case the former was 100% better in G-Ratios that the latter.
The problem of extremely poor performance in dry grinding with sintered aluminous abrasive in the more commonly used vitrified bonds is addressed by U.S. Pat. No. 4,543,107. The inventor discovered that if the viscosity and/or maturing temperature of the bond is properly controlled, then the superior properties of sintered aluminous abrasive are brought out. This was accomplished by reducing the firing temperature (maturing temperature of the bond) to 1100 or less for conventional bonds or 1220.degree. C. or less for the higher viscosity bonds.
While U.S. Pat. No. 4,543,107 has solved the problem of poor dry grinding properties associated with sintered aluminous abrasive bonded with the commonly used vitrified bonds, it has done nothing for the other significant area where the inherent goodness of sintered aluminous abrasives is not observed: that area is in the very important grinding operation called "wet grinding." In this type of operation, the workpiece and the grinding wheel are flooded with a coolant which can be essentially all water but may contain minor quantities of bactericide, antifoaming agents and the like, or, water containing 5-10% of a water soluble oil, or an all oil coolant; the instant invention and this discussion is concerned only with the water based coolants. It is well known that some decrease in grinding quality or G-Ratio is experienced in certain types of grinding, when a given vitrified bonded wheel goes from dry grinding to grinding with water. The drop is much more serious, however, in certain situations, being as large as 90% for vitrified bonded sintered aluminous abrasive wheels. Particularly in the case of wheels made with abrasive made according to the seeded sol gel technique referred to above, the reduction in G-ratio amounts to a loss of essentially all of the inherent superiority of that abrasive as compared to the conventional fused alumina which shows a drop of about 30% if the G-ratio for all infeeds are averaged.
As is also well known in the art, the use of a given vitrified bonded grinding wheel for wet grinding does not always produce results where there is a drop in grinding quality and other aspects of the grinding operation such as power consumption; the coolant in some wet grinding operations can actually cause the grinding quality to increase over that which results when dry grinding. In the case of sintered aluminous abrasives bonded with a conventional vitrified bond, the increase in grinding performance as a result of the coolant doesn't occur or is minimized. In other words, the exposure of the combination of commonly used vitrified bonds and sintered aluminous abrasives to water destroys a major part of the superior properties of that particular abrasive type. It is this very phenomenon with which the present invention is concerned.
Of relevance to the present invention are U.S. Pat. Nos. 1,338,598 and 1,918,312. They are relevant for their teaching of bonding abrasive grain with a frit to form a grinding wheel. The abrasive grain in both patents is the fused alumina type. Frits are well known materials and have been used for many years as enamels for coating, for example, metals and jewelry and for bonding abrasives as evidenced by the foregoing patents. Frit is a generic term for a material that is formed by thoroughly blending several minerals, oxides, and other inorganic compounds, followed by heating the mixture to a temperature at least high enough to melt it; the glass is then cooled and pulverized. There are almost an infinite number of possible frits in view of the numerous combinations of materials and amounts thereof. Some of the more common materials that are used to form frits are: feldspar, borax, quartz, soda ash, red lead, zinc oxide, whiting, antimony trioxide, titanium dioxide, sodium silicofluoride, flint, cryolite, and boric acid. Several of these materials are blended together as powders, fired to fuse the mixture, and the fused mixture is then cooled. The cooled glass is comminuted to a very fine state. It is this final powder that is used to bond abrasive grain to form a grinding wheel.